A non-aqueous secondary battery, as typified by a lithium ion secondary battery, is characterized by a high energy density and thus has been widely used as a power source for portable equipment such as a portable telephone and a notebook personal computer. The capacity of the lithium ion secondary battery is likely to increase further as the performance of the portable equipment becomes higher. Accordingly, research and development is progressing to further improve the energy density of the lithium ion secondary battery.
On the other hand, due to the improved performance of the non-aqueous secondary battery, the non-aqueous secondary battery has recently started to be used as a power source other than that for the portable equipment. For example, the non-aqueous secondary battery has started to be used as a power source for automobiles, motorcycles, movable bodies such as a robot, etc. When the non-aqueous secondary battery is used as a power source for automobiles, motorcycles, movable bodies such as a robot, etc., the capacity needs to be increased even more.
As a measure to increase the capacity of the non-aqueous secondary battery, there is a method for increasing the thickness of an electrode mixture layer. However, such an increase in the thickness of the electrode mixture layer can reduce the capacity during high-current charge and discharge. One reason for this is probably because the number of active materials that are farther away from a current collector increases as the electrode mixture layer becomes thicker, so that the conductivity in the electrode is reduced.
As another measure to increase the capacity of the non-aqueous secondary battery, there is a method for using a high-capacity negative electrode material such as SiOx having a structure in which Si ultrafine particles are dispersed in a SiO2 matrix (see, e.g., Patent Documents 1 to 3). However, this type of high-capacity negative electrode material undergoes a very large volume change during charge and discharge. Therefore, a battery using the high-capacity negative electrode material has a problem that the battery characteristics are sharply reduced by the repetition of charge and discharge.
To solve this problem, e.g., Patent Document 4 proposes a non-aqueous secondary battery that can improve the charge-discharge cycle characteristics by using a combination of a material containing Si and O as constituent elements and graphite.